Abstract The molecular mechanism of clonal dominance in del(5q) MDS Myelodysplastic syndrome (MDS) is a clonal stem cell disease, characterized by ineffective hematopoiesis. Sequence analysis provides direct evidence that almost all bone marrow cells are clonally derived in MDS. How the initiating MDS stem cell outcompetes normal hematopoietic stem cells (HSCs) and grows to become dominant in the neoplasm is poorly understood. Explaining how MDS evolves can help us to develop new strategies to improve the therapy of MDS by targeting early molecular events in HSCs in MDS. Deletion of chromosome 5q [del(5q)] is one of the most common cytogenetic abnormalities in MDS and therapy-related MDS. We found that the expression of FOXM1, a member of the forkhead family of transcription factors, is reduced to approximately 50-60% of normal expression in CD34+ cells from del(5q) MDS patients. Via loss of function studies, we recently identified a previously unrecognized function of Foxm1 in hematopoietic stem and progenitor cells (HSPCs). In contrast to its known function as a pro-proliferation factor in other tissues, conditional deletion of Foxm1 reduces HSC quiescence, leading to disruption of HSC self-renewal. Our preliminary results revealed that Foxm1 haploinsufficiency promoted HSC exit from quiescence but induced HSC expansion with a competitive repopulation advantage. In addition, we identified orphan nuclear receptors as new down-stream targets of Foxm1 in HSPCs. Orphan nuclear receptors are important regulators of HSC quiescence and self-renewal and are recognized as novel tumor suppressors of hematological malignancies. We found that FOXM1 and its downstream targets were all down-regulated in CD34+ HSPCs from del (5q) MDS patients. Thus, we hypothesize that moderate downregulation of FOXM1-mediated pathways plays a critical role in establishing clonal dominance of MDS stem cell in del(5q) MDS patients and that FOXM1 can be targeted for eliminating MDS stem cells in del(5q) patients.To test this hypothesis, we will 1) determine the pathogenic role of Foxm1 downregulation in the development of MDS; 2) investigate the molecular mechanisms that mediate gene dosage-dependent effects of Foxm1 in regulating HSC quiescence, survival and self-renewal; and 3) determine the upstream pathway that regulates Foxm1 expression in HSPCs. We expect that our studies will uncover a dose-dependent role of Foxm1 as a novel critical regulator of HSC maintenance as well as a novel pathogenic role of Foxm1 in the development of MDS. We expect to identify novel molecular mechanisms that regulate HSC quiescence, survival and self-renewal. These studies will provide mechanistic insights into the acquisition of clonal advantage by MDS stem cells at early stages of del(5q) MDS. Our studies likely will lead to the identification of more effective therapeutic strategies for eliminating disease-propagating cells at early stages of del(5q) MDS by targeting FOXM1.